An electronic device such as a GaN based semiconductor device, in which a GaN layer and an AlGaN layer are grown sequentially over a substrate and the GaN layer is used as a channel layer, has been actively developed. A sapphire substrate, a SiC substrate, a GaN substrate, a silicon substrate and the like are used as a substrate on which the GaN layer is deposited.
The GaN layer has an energy bandgap of 3.4 eV that is larger than that of 1.4 eV of the GaAs layer, and is expected to operate in a higher voltage and withstand a higher voltage.
Japanese Patent Application Publication 2006-114653-A, for example, discloses the GaN High Electron Mobility Transistor (HEMT) as a high withstand-voltage GaN electronic device.
The GaN-HEMT used as an amplifier has a load line characteristic so as to provide high load impedance, and hence can operate very efficiently.
Since the GaN-HEMT exhibits a high power density, and hence a chip size of the GaN-HEMT can be reduced, and a power loss in the amplifier can be reduced. Thus, a cost for managing a small amplifier, which provides large load impedance and easiness of impedance matching, can be reduced.
Because of such advantages, recently the GaN-HEMT is being developed intensively for a communication system at a higher frequency in a milimeter-wavelength band.
The conventional GaN-HEMT has a structure shown in FIG. 9, for example.
In FIG. 9, an AlN layer 102, a GaN layer 103, an n-type AlGaN layer 104, and an n-type GaN layer 105 are formed sequentially on a SiC substrate 101. In this case, the AlN layer 102 and a lower portion of the GaN layer 103 form a buffer layer.
Also, Ohmic-contacts of a source electrode 107s and a drain electrode 107d which are disposed at a distance are formed on the n-type AlGaN layer 104. Also, Schottky-contact of a gate electrode 108 which is disposed at distances between the source electrode 107s and the drain electrode 107d is formed on the n-type GaN layer 105.
Also, a silicon nitride layer 106 is formed as a protection insulating film on the n-type GaN layer 105 and between the source electrode 107s and the drain electrode 107d. 